Holey Ruthenium Nanosheets with Moderate Aluminum Modulation toward Hydrogen Evolution

Article abstract of DOI:10.1021/acs.inorgchem.9b01150

Theoretical calculations reveal that aluminum (Al) doping can effectively modulate the electronic structures of 2D ruthenium (Ru) catalysts. Moderate Al incorporation can endow Ru nanosheets with more delocalized electrons and optimal hydrogen adsorption Gibbs free energy, providing opportunities to achieve improved hydrogen evolution performance. Thus, Al-doped Ru nanosheets have been synthesized by a solvothermal strategy, in which they exhibit holey nanosheet structures and have more active sites exposed on the basal plane. The characterizations unraveling the Ru structure can be well maintained, and electrochemical measurements confirm the appropriate amount of Al modulation that can extremely enhance its hydrogen evolution activity.

Full text of DOI:10.1021/acs.inorgchem.9b01150

Communication
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Holey Ruthenium Nanosheets with Moderate Aluminum Modulation
toward Hydrogen Evolution
†
,‡
†,‡
†
§
†
,†
Hang Zhang, Qilong Liu, Jie Xu, Lingzhi Wei, Qiangchun Liu, and Xiangkai Kong*
†Key Laboratory of Green and Precise Synthetic Chemistry and Application, Ministry of Education, Huaibei Normal University,
Huaibei, Anhui 235000, P. R. China
§
Center of Modern Experiment and Technology, Anhui University, Hefei, Anhui 230601, P. R. China
*
S Supporting Information
In general, the intrinsic activity of a single active site, the
amount of accessible active sites on the surface, and the
conductivity are three indicators determining the activity of
electrocatalysts. Given that metallic Ru normally has good
conductivity, enhancing the catalytic capability and exposing
more active sites at the same time should be a good choice for
the design of new and effective Ru-based catalysts. As is
known, aluminum (Al) modulation has rarely been reported as
an efficient strategy on electrocatalysts, especially for HER. In
this study, we find that the introduction of Al can efficiently
modify the surface electronic states of Ru nanosheets, affording
them with more delocalized electrons and optimal hydrogen
adsorption. Meanwhile, the 2D nanosheet morphology can be
well maintained by moderate Al doping. Moreover, holey
structures are generated on the nanosheets, which benefit the
exposure of active sites and facilitate the diffusion of reactants
and products. Therefore, the as-prepared Ru−Al alloys can
exhibit extremely improved electrocatalytic kinetics toward
HER.
ABSTRACT: Theoretical calculations reveal that alumi-
num (Al) doping can effectively modulate the electronic
structures of 2D ruthenium (Ru) catalysts. Moderate Al
incorporation can endow Ru nanosheets with more
delocalized electrons and optimal hydrogen adsorption
Gibbs free energy, providing opportunities to achieve
improved hydrogen evolution performance. Thus, Al-
doped Ru nanosheets have been synthesized by a
solvothermal strategy, in which they exhibit holey
nanosheet structures and have more active sites exposed
on the basal plane. The characterizations unraveling the
Ru structure can be well maintained, and electrochemical
measurements confirm the appropriate amount of Al
modulation that can extremely enhance its hydrogen
evolution activity.
he rapid depletion of traditional fossil fuels and their
T
pollution of the environment have driven us to explore
Density functional theory (DFT) calculations are performed
on pristine and Al-doped Ru nanosheets to directly visualize
and understand the effect of Al incorporation on electronic
structures and the HER performance. Hexagonal Ru(001) is
constructed with the optimized structure shown in Figure 1a,
and a series of Ru−Al alloy models with different amounts of
1
appropriate alternatives. Molecular hydrogen is a central
−
1
energy carrier owing to its high energy density (120 MJ kg )
2
,3
and carbon-free feature. Electrocatalytic water splitting is
efficient to produce hydrogen (H O → H + O ), and
2
2
2
exploring efficient and inexpensive catalysts for hydrogen
+
−
evolution reaction (HER; 2H + 2e → H ) is highly
2
4
,5
desired. Although platinum (Pt)-based materials can exhibit
excellent performance for HER, their high cost and limited
reserve prevent their large-scale applications.
6
Compared with Pt, ruthenium (Ru) is much cheaper and
7
,8
has comparable activity toward hydrogen generation.
Structure engineering to obtain a particular morphology,
such as 2D nanosheets, can expose more active sites and
9
reduce the amount of Ru usage. Moreover, introducing low-
cost metals as dopants to partially replace Ru can further
decrease the catalyst cost. Meanwhile, the selection of
appropriate dopants can modify its surface electronic structure,
facilitating hydrogen adsorption and thus accelerating hydro-
gen generation. For instance, Kim and co-workers have
designed hybrid nickel (Ni)- and Ru-based architectures.
The synergetic and antagonistic effects of the hybrid material
make it a superior bifunctional electrocatalyst for water
Figure 1. DFT calculations for pristine and Al-doped Ru nanosheets:
(a) optimized 2D Ru structure; (b) charge-density distribution on the
surface, with hydrogen adsorption sites marked; (c and d) DOSs
(
inset: enlarged images for states around the Fermi level); (e) HER
free-energy diagrams.
1
0
splitting. Chen and co-workers reported a cobalt (Co)
modulation effect for Ru−Co nanoalloys and revealed their
11
advances for hydrogen production. Yet, exploring Ru-based
catalysts with improved HER behavior is still challenging.
Received: April 19, 2019
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b01150
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
Al incorporation are considered. Figure 1b depicts the surface
static electron distributions, where Ru0.94Al0.06 exhibits more
delocalized electron distribution than pristine Ru, which is
beneficial to activating and enriching the electrically active sites
12
and further accelerating the HER process. Densities of states
DOSs) are also calculated in Figure 1c,d. Compared with
(
pristine Ru, Ru0.94Al0.06 possesses more states at edges in the
valence band, which benefits electron transfer from the catalyst
+
surface to the adsorbed H , expediting its reduction.
Meanwhile, compared with Ru0.97Al0.03 (light-doping structure)
and Ru0.92Al0.08 (heavy-doping structure), Ru0.94Al0.06 shows
many more states around the Fermi level, indicating that it has
the highest concentration of charge carriers and superior
electronic conductivity. After that, three sites, as marked in
Figure 1b, are considered for hydrogen adsorption, and the
hollow site (H) exhibits the most stable configuration. Figure
1
e illustrates the calculated free-energy diagrams. Remarkably,
Al incorporation can effectively modify its |ΔG |. The |ΔG |
value of Ru Al is 0.2 eV, significantly smaller than that of
H*
H*
0
.94 0.06
pristine Ru (0.43 eV) and both heavy and light Al-doped Ru
models. |ΔG | is a catalytic descriptor for the HER, and the
H*
13
optimal value should be close to 0. Henceforth, it is
anticipated that moderate Al doping can provide higher HER
activity on Ru nanosheets.
Figure 2. Characteristics of Ru Al: (a and b) TEM images; (c)
HRTEM image; (d) SAED pattern; (e) STEM image and
corresponding elemental mapping images.
3
On the basis of these theoretical predictions, we have tried
to prepare Ru nanosheets with optimized Al incorporation via
a facile solvothermal reaction. The detailed preparation can be
found in the Supporting Information. Figure S1 shows the X-
ray diffraction (XRD) patterns measured on these samples.
Four peaks appearing at 38.4, 42.2, 44.1, and 69.4° are indexed
to the (100), (002), (101), and (110) planes of the hexagonal-
close-packed Ru metal (PDF 06-0663), suggesting that they
are in the metallic Ru phase (Figure S1a). Clearly, there is no
nanoflake morphology. Moreover, holey structures are also
obtained on Ru Al and RuAl, in association with that observed
5
on Ru Al, suggesting that these holes are generated by Al
3
dopants. Al is a typical amphoteric material, with radii (118
pm) comparable to that of Ru (125 pm); thus, it can partially
substitute Ru and act as a holey building block during the
solvothermal reaction.
To acquire the microstructural information, high-resolution
TEM (HRTEM) measurements were carried out. Figure 2c
big difference between Ru Al and pristine Ru, implying that Al
3
is appropriately incorporated for Ru Al, without a significant
reveals a lattice distance of 0.208 nm on Ru Al, which is
3
3
crystal structure change occurring in the Ru matrix. We
ascribed to the (002) crystal plane of the Ru crystal. The
decreased d-space distance compared to the pristine Ru(002)
plane (Figure S3c,d) can be explained by Al intercalation.
Furthermore, Figure 2d illustrates the selected-area electron
diffraction (SAED) pattern, where two diffraction rings are
speculate that the formation of Ru Al experiences the same
3
attachment-growing process as that for pristine Ru nano-
9
sheets. However, when more Al precursors are employed
(
heavy doping), some new peaks appear (Figure S1b), which
1
4
are attributed to the diffraction peaks of AlO(OH), as indexed
to PDF 21-1307. In this stage, this material becomes a
composite of Ru and AlO(OH) rather than the formation of
pure Ru−Al alloys. The energy-dispersive X-ray (EDX)
measurements illustrate that the atomic ratios between Al
and Ru are 0.18, 0.30, and 1.03 for Ru Al, Ru Al, and RuAl,
observed, demonstrating its polycrystalline feature. The two
measured rings indicate 0.206 and 0.135 nm lattice distances,
corresponding to the (002) and (110) crystal planes of Ru,
respectively, which are also observed as diffraction peaks in the
XRD pattern. Moreover, the well distributions of Al and Ru on
the nanosheet are observed from the scanning TEM (STEM)−
EDX mapping analysis (Figure 2e), verifying the successful Al
incorporation on Ru nanosheets. Atomic force microscopy
(AFM) measurement discloses the height profile of about 1.6
nm for these nanosheets, corresponding to 6−8 atomic layers
stacking along the c axis with the (001) planes as the basal
planes (Figure S5).
5
3
respectively (Figure S2). Therefore, controlling the amount of
Al intercalation is pivotal to achieving ideal alloy materials with
enhanced catalytic behavior.
Transmission electron microscope (TEM) characterizations
were performed to unravel the morphology of the Ru Al
3
catalyst (Figure 2a,b). Flat nanosheet structures without any
significant strips and crimps are found in these TEM images.
Most of the nanosheets are dozens of nanometers in size and
are stacked on each other. Interestingly, some holes are
observed on the nanosheet surface, which are beneficial for
more active sites exposed on the basal plane. Meanwhile, these
generated holes can facilitate the diffusion of reactants and
products and thus accelerate the HER process. For
X-ray photoelectron spectroscopy (XPS) spectra have been
applied to investigate the surface chemical states and elemental
compositions. As shown in Figure S6a, two distinct peaks
around 461.7 and 483.6 eV are attributed to Ru 3p1/2 and Ru
3p3/2, respectively, and both can be seen on pristine Ru and
15
Ru Al. After Al incorporation, these two peaks move 0.4 eV
3
toward the lower-energy direction, suggesting its modulation
essence on Ru nanosheets. Figure S6b displays the high-
resolution Ru 3p spectra, and they are deconvoluted into
comparison, pristine Ru, Ru Al, and RuAl are also measured,
5
with their TEM images given in Figures S3 and S4,
respectively. Generally, pure Ru exhibits a larger sheetlike
3
/2
0
IV
16
Ru and Ru , located at 461.3 and 463.3 eV, respectively.
The intensity ratio of Ru to Ru will change from 3.22 to
0
IV
structure, while Ru Al and RuAl display relatively smaller
5
B
DOI: 10.1021/acs.inorgchem.9b01150
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
.15, implying that more reduced Ru states are induced via Al
doping, in line with the negative shifts for the above Ru 3p
peaks. This could be explained by their difference in
electronegativity. Because Ru (χ = 2.2) has a larger
electronegativity than Al (χ = 1.5), more electrons will transfer
from Al to Ru after its incorporation. Additionally, the Al 2p
Communication
4
incorporation. The elecrochemical surface area is determined
by CO stripping measurement, with the oxidation peak area for
the preadsorbed CO molecules shown in Figures 3e and S8.
Ru Al possesses a larger electrochemically active area than
3
pristine Ru, about 1.22 times that for the pristine counterpart,
confirming that more active sites are generated by Al. When
normalized by an electrochemically active surface area, the
peak around 74 eV is obtained on Ru Al in Figure S6c, further
3
confirming the successful synthesis of an Al-doped Ru
area-specific HER current density of Ru Al is calculated to be
3
1
7,18
catalyst.
The 3+ state of Al may be caused by the
5.1 and 7.2 times those of pristine Ru, at −0.05 and −0.10 V
versus reversible hydrogen electrode, respectively, verifying the
enhanced HER kinetics via Al modulation. In addition, the
durability is also estimated. There is a negligible difference
between the LSV curves measured initially and after 3000
cycles, and the overpotential recorded at 10 mA cm− exhibits
a slight change during the stability measurement (Figure 3f).
transferred electrons to Ru and partial oxidation on the surface.
After that, the electrocatalytic performance is evaluated
19,20
using a three-electrode system in 0.5 M H SO .
Figure 3a
2
4
2
TEM image of Ru Al after the durability test also shows a
3
holey nanosheet structure as before (Figure S9a). Moreover,
the EDX measurement, XRD pattern, and elemental mapping
images shown in Figure S9b−d also indicate the good stability
of the Al-modulated Ru catalyst.
In summary, on the basis of the combination of DFT
calculations and experimental measurements, we find that Al
engineering can effectively regulate the electronic structure of
Ru nanosheets and further improve their HER activity. The
enhanced catalytic performance should be ascribed to (1) the
induced more delocalized electrons caused by Al incorpo-
ration, (2) the optimized hydrogen adsorption Gibbs free
energy, and (3) the formed holey structure with more exposed
active sites. The synthesis of Al-incorporated Ru nanosheets is
facial and adds a new component to the modulation strategies
for obtaining high-performance functional materials.
Figure 3. Electrochemical characterizations: (a) LSV curves; (b)
required overpotentials for 10 and 50 mA cm ; (c) Tafel slopes; (d)
EIS at an overpotential of 150 mV; (e) CO stripping measurement;
−2
(
1
f) LSV curves for the durability evaluation. Inset: Overpotentials for
0 mA cm− recorded at each 500 cycles.
2
shows the recorded linear-sweep voltammetry (LSV) curves
measured on these prepared samples. Meanwhile, commercial
Pt/C and bare glass carbon are also performed for comparison
ASSOCIATED CONTENT
■
*
S
Supporting Information
(Figure S7). Pristine Ru nanosheets exhibit relatively moderate
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b01150.
activity toward hydrogen generation. With an increase in the
amount of the Al component, the measured current density
will gradually increase. However, when too much Al is
employed, its catalytic performance will significantly decrease.
Experimental and calculated methods, XRD patterns,
EDX spectra, TEM images, AFM characterizations, XPS
data, LSV measurements, and CV curves (PDF)
Among them, Ru Al shows the highest HER activity with a
3
−
2
small onset overpotential of 10 mV. To reach a 10 mA cm
current density, Ru Al needs an overpotential of 79 mV, much
3
smaller than the 164, 200, and 234 mV required for Ru Al,
5
AUTHOR INFORMATION
pristine Ru, and RuAl, respectively (Figure 3b). In addition,
the current densities of 20 and 50 mA cm− can be obtained at
the overpotentials of 109 and 142 mV, respectively. After that,
the kinetics of HER is studied by calculating the Tafel slope. In
general, the rate-determining step consists of three basic steps;
■
*
2
Corresponding Author
E-mail: kxk@chnu.edu.cn (X.K.).
ORCID
+
−
Xiangkai Kong: 0000-0002-0552-5557
Author Contributions
These authors contributed equally.
(
1) the Volmer reaction step H + e → H ; (2) the
ads
+
−
Heyrovsky reaction step Hads + H + e → H ; (3) the Tafel
2
21
‡
reaction step Hads + Hads → H2. The linear Tafel plots are
obtained from the low overpotential region of the polarization
Notes
curves. As shown in Figure 3c, Ru Al owns a Tafel slope of 81
3
The authors declare no competing financial interest.
−
1
−1
mV dec , smaller than pristine Ru (94 mV dec ), Ru Al (89
5
−
1
−1
mV dec ), and RuAl (98 mV dec ). This indicates that
moderate Al doping can effectively accelerate the HER process
of Ru, without changing the Volmer−Heyrovsky reaction
mechanism. Meanwhile, this enhancement is also supported by
the electrochemical impedance spectroscopy (EIS) measure-
ment. Figure 3d illustrates their corresponding Nyquist plots,
with a dramatic declined charge-transfer resistance of 59.7 Ω
ACKNOWLEDGMENTS
■
The work was supported by the National Natural Science
Foundation of China (Grant 51602116), Natural Science
Foundation of Anhui Province (Grant 1708085QB40), and
China Postdoctoral Science Foundation (Grant
2016M600492) (to X.K.). The calculations were completed
on the supercomputing system in the Supercomputing Center
of University of Science and Technology of China.
for Ru Al relative to others, suggesting the enhanced interfacial
3
resistance and improved conductivity caused by moderate Al
C
DOI: 10.1021/acs.inorgchem.9b01150
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
(
20) Yang, L.; Zhou, H.; Qin, X.; Guo, X.; Cui, G.; Asiri, A.; Sun, X.
REFERENCES
■
Cathodic electrochemical activation of Co3O4 nanoarrays: a smart
strategy to significantly boost the hydrogen evolution activity. Chem.
Commun. 2018, 54, 2150−2153.
(
1) Chu, S.; Cui, Y.; Liu, N. The path towards sustainable energy.
Nat. Mater. 2017, 16, 16−22.
(
2) Thomas, G. Overview of Storage DevelopmentDOE Hydrogen
Program; Sandia National Laboratories, 2000.
3) Tan, Y.; Wang, H.; Liu, P.; Shen, Y.; Cheng, C.; Hirata, A.;
(21) Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H. MoS2
nanoparticles grown on graphene: an advanced catalyst for the
(
hydrogen evolution reaction. J. Am. Chem. Soc. 2011, 133, 7296−
Fujita, T.; Tang, Z.; Chen, M. Versatile nanoporous bimetallic
phosphides towards electrochemical water splitting. Energy Environ.
Sci. 2016, 9, 2257−2261.
7299.
(
4) Wang, Z.; Du, H.; Liu, Z.; Wang, H.; Asiri, A.; Sun, X. Interface
engineering of a CeO2-Cu3P nanoarray for efficient alkaline hydrogen
evolution. Nanoscale 2018, 10, 2213−2217.
(
5) Sun, Z.; Yuan, M.; Yang, H.; Lin, L.; Jiang, H.; Ge, S.; Li, H.;
Sun, G.; Ma, S.; Yang, X. 3D Porous Amorphous γ-CrOOH on Ni
Foam as Bifunctional Electrocatalyst for Overall Water Splitting. Inorg.
Chem. 2019, 58, 4014−4018.
(
6) Feng, J. X.; Xu, H.; Dong, Y. T.; Lu, X. F.; Tong, Y. X.; Li, G. R.
Efficient Hydrogen Evolution Electrocatalysis Using Cobalt Nano-
tubes Decorated with Titanium Dioxide Nanodots. Angew. Chem., Int.
Ed. 2017, 56, 2960−2964.
(
7) Zhang, L.; Wang, Y.; Li, J.; Ren, X.; Lv, H.; Su, X.; Hu, Y.; Xu,
D.; Liu, B. Ultrasmall Ru Nanoclusters on Nitrogen-Enriched
Hierarchically Porous Carbon Support as Remarkably Active Catalysts
for Hydrolysis of Ammonia Borane. ChemCatChem 2018, 10, 4924−
4
(
930.
8) Xu, C.; Ming, M.; Wang, Q.; Yang, C.; Fan, G.; Wang, Y.; Gao,
D.; Bi, J.; Zhang, Y. Facile synthesis of effective Ru nanoparticles on
carbon by adsorption-low temperature pyrolysis strategy for hydrogen
evolution. J. Mater. Chem. A 2018, 6, 14380−14386.
(
9) Kong, X.; Xu, K.; Zhang, C.; Dai, J.; Norooz Oliaee, S.; Li, L.;
Zeng, X.; Wu, C.; Peng, Z. Free-Standing Two-Dimensional Ru
Nanosheets with High Activity toward Water Splitting. ACS Catal.
2
(
016, 6, 1487−1492.
10) Marques Mota, F.; Choi, C. H.; Boppella, R.; Lee, J. E.; Kim, D.
H. Arising synergetic and antagonistic effects in the design of Ni- and
Ru- based water splitting electrocatalysts. J. Mater. Chem. A 2019, 7
(
2), 639−646.
(
11) Su, J.; Yang, Y.; Xia, G.; Chen, J.; Jiang, P.; Chen, Q.
Ruthenium-cobalt nanoalloys encapsulated in nitrogen-doped gra-
phene as active electrocatalysts for producing hydrogen in alkaline
media. Nat. Commun. 2017, 8, 14969.
(
12) Liu, Q.; Wei, L.; Liu, Q.; Chen, G.; Kong, X. Anion Engineering
on 3D Ni3S2 Nanosheets Array toward Water Splitting. ACS Appl.
Energy Mater. 2018, 1, 3488−3496.
(
13) Parsons, R. The rate of electrolytic hydrogen evolution and the
hear of adsorption of hydrogen. Trans. Faraday Soc. 1958, 54, 1053−
063.
14) Ambare, R. C.; Bharadwaj, S. R.; Lokhande, B. J. Non-aqueous
1
(
route spray pyrolyzed Ru:Co3O4 thin electrodes for supercapacitor
application. Appl. Surf. Sci. 2015, 349, 887−896.
(
15) Hassan, H. K.; Atta, N. F.; Hamed, M. M.; Galal, A.; Jacob, T.
Ruthenium nanoparticles-modified reduced graphene prepared by a
green method for high-performance supercapacitor application in
neutral electrolyte. RSC Adv. 2017, 7, 11286−11296.
(
16) Raman, R. K.; Shukla, A. K.; Gayen, A.; Hegde, M. S.; Priolkar,
K. R.; Sarode, P. R.; Emura, S. Tailoring a Pt−Ru catalyst for
enhanced methanol electro-oxidation. J. Power Sources 2006, 157 (1),
4
(
5−55.
17) Liu, Y. J.; Deng, X.; Jia, L.; Huang, W. Investigation of copper
precursors in the synthesis of higher alcohols from syngas over
CuZnAl catalysts without promoters. Phys. Chem. Chem. Phys. 2018,
2
(
0, 18790−18799.
18) Huestis, P.; Pearce, C. I.; Zhang, X.; N’Diaye, A. T.; Rosso, K.
M.; LaVerne, J. A. Radiolytic stability of gibbsite and boehmite with
adsorbed water. J. Nucl. Mater. 2018, 501, 224−233.
(
19) Liu, Q.; Zhang, H.; Xu, J.; Wei, L.; Liu, Q.; Kong, X. Facile
Preparation of Amorphous Fe−Co−Ni Hydroxide Arrays: A Highly
Efficient Integrated Electrode for Water Oxidation. Inorg. Chem. 2018,
5
7, 15610−15617.
D
DOI: 10.1021/acs.inorgchem.9b01150
Inorg. Chem. XXXX, XXX, XXX−XXX